It is known in the art of broaching to provide for machines which utilize an endless chain for carrying a series of tools into contact with one or more workpieces. Additionally, it is known to utilize machines of this type for producing spur and helical gears. Examples of early machine designs for producing gears with tools carried on articulated links making up an endless chain are disclosed in U.S. Pat. Nos. 1,468,393; 1,469,602; 2,475,690; 2,692,537; and 2,749,804.
Machines utilizing an endless chain for carrying a plurality of tools offer a potential for very high speed production of whatever shape of workpiece is being formed by the tools. In fact, in the case of gear production, it is contemplated that stacks of gear blanks may be worked simultaneously with machinery of this type, rather than one at a time as has been the case with many other types of gear forming machines. However, the requirements for gear manufacture are far more severe than those for other forms of broaching, and therefore, machines of this type have not been, as far as is known, commercially acceptable for high production gear manufacturing needs. The forming of gear tooth profiles on a work blank requires precise control of an engagement between a workpiece and each cutting tool, and this requirement demands, in turn, a very rigid machine which maintains precise placement of workpiece and tool under a working load and for a sustained operation of the machine. Since an endless chain is by its very nature not as rigid as other forms a tool-holding equipment, it has been a problem in this art to design and manufacture an endless chain type of machine having the capability of high production manufacture of precision gears.
One of the features of the present invention is to provide for a means for effectively tightening and stretching a portion of an endless chain in the zone in which its tools make contact with one or more workpieces. Prior efforts in this regard (as shown in U.S. Pat. No. 2,475,690 and 2,749,804, for example) have suggested the placement of a gear for driving the endless chain in a position immediately behind the chain and at a point where the chain is engaging a workpiece. The apparent reason for such placement was to provide a firm drive for the chain in its zone of heaviest load during a cutting operation so as to establish, as much as possible, a precise and known relationship between the cutting tools of the chain and the workpiece.
In contrast to the prior art suggestion of backing the chain with a drive gear at its zone of contact with a workpiece, the present invention provides for a pair of spaced apart driven members in positions which are upstream and downstream from the zone of contact of the endless chain with the workpiece. One of the driven members is driven so as to advance the endless chain, while the other is effectively driven by the endless chain. In this sense the "driven members" are each driven in a different way, and they could just as well be called "idlers" or "gears", however, for purposes of this discussion reference will be made throughout to "driven members". The driven members are designed so that they can be adjusted and fixed relative to each other in a way which tends to "stretch" and remove backlash from articulated links of the portion of the chain being driven by and between the spaced driven members. This action effectively tightens the chain between the two driven members to thereby remove any looseness in its links and points of articulation and to make it more rigid. The tightened chain is less likely to chatter or deflect in the cutting zone, and this permits a better control of tooth profile in the manufacture of precision gears. The spacing of the driven members is sufficient to include several articulated links of the endless chain so that an entire section of chain, for its entire width, can be made effectively rigid to accommodate a stack of gears in the cutting zone and to provide a rigid face across which one or more gears can be translated during a cutting operation. Thus, the invention has an effect of making a relatively large portion of an endless chain sufficiently rigid to permit precision production of multiple numbers of workpieces or to accommodate greater relative movement between even a single workpiece and the cutting tools of such a machine, than has been attained in the past.
In addition, the adjustable driven members of this invention function to remove backlash from the relatively small gears making up the drive train for the endless chain. This is important because it permits the use of smaller drive gears, within the confines of the chain itself, than would otherwise be possible.
In accordance with a preferred embodiment of the invention, the tightening means for stiffening the endless chain includes a pair of spaced drive members which can be rotated and fixed relative to each other in a driving relationship with the endless chain so that articulated links of the endless chain are effectively stretched and tightened in a flat plane defined between the driven members. A common driving means serves to move both of the driven members is synchronism with each other.
Since the spaced driven members which are used for tightening the endless chain do not function, in the same sense as certain prior art arrangements, to support the endless chain in its cutting zone, a series of rollers are provided in the cutting zone for firmly backing the endless chain as it engages one or more workpieces. These rollers and their specific functions will be described herein in combination with the tightening means of this invention and are more fully described in my copending patent application Ser. No. 930,779, Aug. 2, 1978, filed even date herewith under title of "ENDLESS CHAIN APPARATUS FOR FORMING SPUR AND HELICAL GEARS WITH MEANS FOR CONTROLLING CHAIN".
These and other features and advantages of the present invention will be discussed in further detail below. In that discussion reference will be made to the accompanying drawings as briefly described below.